Why do we always subtract and then later on always add seasonal corrections to get height of tide at a secondary port?

This question is regarding navigation so it is related to science in a way.

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Depending on the time of the year, the hemisphere you are in may be closer or farther away from the sun.  This "seasonal" time could have an influence on the height of the high tide and the lowness of the low tide.  Also, the positioning of the Earth between the sun and the moon can produce higher than normal high tides, called spring tides.  The positioning is that of 180 degrees, with the sun, the Earth, and the moon in a straight line pattern.  A lower high tide can also be produced, cancelling out the effect that normally takes place by having the sun, the Earth and the moon at 90 degree angles to each other.

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